1. Field of the Invention
The present invention relates to a protection relay system for protecting a power system and, particularly, to a current differential relay device for protecting a power transmission line based on the principle of current differential.
2. Prior Art
FIG. 10 illustrates a constitution example in which a current differential relay device is applied for protecting a power transmission line constituting a power system. A current differential relay system shown in FIG. 10 includes a pair of current differential relay devices 2 and 3 provided at terminals 1A and 1B of the power transmission line 1. Current values at the terminals 1A and 1B of the power transmission line 1 are detected by current detectors 4 and 5, and the detected current values are sent to the current differential relay devices 2 and 3 at the terminals, and are sampled and are converted into digital values. Here, the sampled current values are used by the current differential relay devices 2 and 3 at the terminals, and are further sent to the current differential relay devices 3 and 2 at the other terminals relative to each other through signal transmission lines 12 and 13 provided between the current differential relay devices 2 and 3. Thus, the current differential relay device 2(3) executes the operation for current differential protection based on a current value at its own terminal 1A(1B) and a current value at the other terminal 1B(1A), and generates an output signal based on the result of operation. An accident that happens to occur on the power transmission line is detected based on the current differential that abnormally increase at both terminals, and at least one breaker in the power transmission line 1 is tripped to remove accident.
The current values used by the current differential relay devices 2 and 3 for executing the operation for current differential protection must be those that are sampled at the same time by the two current differential relay devices 2 and 3. For this purpose, it is essential that the current values used by the two current differential relay devices 2 and 3 for operating the current differential protection must be in synchronism with each other. It therefore becomes necessary to control the sampling synchronization in order to eliminate an error in synchronizing the sampling between the current differential relay devices 2 and 3.
A conventional operation for synchronizing the samplings will be described with reference to FIG. 11. In FIG. 11, the current differential relay devices 2 and 3 send digital transmission signals TX2 and TX3 to the other terminals in synchronism with a sampling time TS2 or TS3 at its own terminal. If the transmission of transmission signals TX2 and TX3 from one terminal to the other terminal involves a delay component due to the transmission lines or a transmission delay time td due to delay in the relay devices, then, the transmission signals TX2 and TX3 are received at its own terminal after the passage of the time td from a reference time at which the transmission signal was transmitted from the other end. Here, the times until the transmission signals are received as measured using the sampling time TS2 or TS3 at its own terminal as a reference are regarded to be reception times t2 and t3. It is desired that the two current differential relay devices 2 and 3 are in synchronism with each other and perform the sampling operations at the same time. However, an error occurs in synchronizing the samplings between the current differential relay devices 2 and 3. FIG. 11 illustrates a case where an error xcex94T exists in synchronizing the samplings between the sampling time TS2 of the current differential relay device 2 and the sampling time TS3 of the current differential relay device 3.
As will be obvious from FIG. 11, the following relations (1) and (2) hold among the reception times t2, t3, transmission delay time td and error xcex94T in synchronizing the samplings. From the relations (1) and (2), the error xcex94T in synchronizing the samplings can be operated according to a relation (3). One or both of the current differential relay devices 2 and 3 control the synchronization of samplings by so adjusting the oscillation frequencies of the sampling synchronization oscillators incorporated therein that the operated error xcex94T becomes zero in synchronizing the samplings.
t2=td+xcex94Txe2x80x83xe2x80x83(1)
t3=tdxe2x88x92xcex94Txe2x80x83xe2x80x83(2)
xcex94T=(t2xe2x88x92t3)/2xe2x80x83xe2x80x83(3)
To operate the error xcex94T in synchronizing the samplings, it is essential that the times t2 and t3 of receiving data from the other terminals as measured with the sampling time at its own terminal as a reference are constant at every period For this purpose, a multiple relationship must exist between the sampling frequency and the rate of transmitting the transmission data. When the multiple relationship does not exist between the sampling frequency and the rate of transmitting the transmission data, the times of receiving data from the other terminals deviate as measured with the sampling time as a reference, and the result of operating the error xcex94T in the sampling period does not become constant at every period and the sampling times can no longer be brought into synchronism with each other.
For example, when the sampling frequency is 720 Hz and the rate of transmission is 54 kbps (kilobits per second), there exists a multiple relationship (54000/720=75) between the sampling frequency and the rate of transmission. Upon periodically sending a data of 75 bits per a sampling period, the times of receiving data from the other terminals become constant as measured with the sampling time as a reference, and the sampling times can be brought into synchronism with each other.
However, when the sampling frequency is 720 Hz and the rate of transmission is, for example, 64 kbps, there does not exist a multiple relationship between the sampling frequencyand the rateof transmission (64000/720=88.88 . . . ) If the data of 88 bits per a sampling period is periodically transmitted, an error corresponding to 0.88 . . . bits occur per a sampling period, and the times for receiving data from the other terminals do not become constant as measured with the sampling time as a reference. According to the prior art, therefore, it is not allowed to bring the sampling timings into synchronism between the current differential relay devices at both terminals.
It is an object of the present invention to provide a current differential relay device capable of stable accomplishing the synchronism of samplings by nearly constantly holding the times for receiving data from the other terminals with the sampling time as a reference even when a multiple relationship does not exist between the sampling frequency and the rate of transmission.
In order to accomplish the above object according to the current differential relay device of the present invention, the terminal currents of the power system are sampled at the same time maintaining a predetermined period, and the protection operation is executed by using the sampled current data, wherein provision is made of means for inserting dummy bits among the fixed frame lengths of transmission signals so that, even when there exists no multiple relationship between the sampling frequency and the rate of transmission, the length of transmission data is adjusted by the insertion of the dummy bits to bring the sampling times into synchronism.
There are further provided means for-accomplishing synchronism between both terminals with the fixed frame length of the transmission signal as a unit, and means for accomplishing synchronism of timings for inserting the dummy bits between both terminals, thereby to adjust the length of transmission data by inserting the dummy bits and to bring the sampling times into synchronism even when there exists no multiple relationship between the sampling frequency and the rate of transmission.
Provision is made of means for inserting dummy bits in synchronism with timings for sending a transmission data of a predetermined period to synchronize the samplings, whereby the length of transmission data is adjusted by the insertion of the dummy bits to synchronize the sampling timings even when there exists no multiple relationship between the sampling frequency and the transmission frequency.
Provision is further made of means for accomplishing the synchronism between both terminals with a fixed frame length of a transmission signal as a unit, means for picking up clocks from the transmission signals, and means for producing sampling synchronizing signals based on the clocks that are picked up, in order to adjust the length of transmission data by inserting dummy bits and to synchronize the sampling timings based on the signals that are received or the clocks that are picked up even when there exists no multiple relationship between the sampling frequency and the transmission frequency.
Provision is further made of means for accomplishing the synchronism between both terminals with a fixed frame length of a transmission signal as a unit, means for receiving clocks from a communication unit, and means for producing sampling synchronizing signals based on the clocks that are received, in order to adjust the length of transmission data by inserting dummy bits and to synchronize the sampling timings based on the clocks output from the communication unit even when there exists no multiple relationship between the sampling frequency and the transmission frequency.
Provision is further made of means for producing dummy bits in a number of n or n xc2x11, in order to adjust the length of transmission data by inserting dummy bits and to synchronize the sampling timings even when there exists no multiple relationship between the sampling frequency and the transmission frequency.